Today, membranes are widely and practically applied to produce potable water from sea, to clean industrial effluents and recover valuable constituents, to concentrate, purify or fractionate macromolecular mixtures in the food and drug industries, and to separate gases and vapors. They are also key components in energy conversation systems, and in artificial organs and drug delivery devices. Their widespread use in separations has, however, been limited by the difficulty of preparing membranes with the desirable combination of high selectivity, which yields high product purity and low operating costs, and high permeability, which reduces membrane area and capital cost, as well as the high membrane flux. Thus high membrane flux is the key performance criterion that determines the cost of a membrane system. Unfortunately, as the selectivity of conventional polymer membrane materials increases, permeability invariably decreases and vice versa; and as decreasing the thickness to increase the flux, the stability dramatically decreased. Attempts to overcome the first fundamental limitation have explored the addition of micron-sized porous zeolite particles to organic polymers in the hope of combining the mechanical elasticity and processability of polymers with the strong size selectivity characteristic of spatially well-defined zeolite pores (Lai, Z. P. et al, 2003). Commercialization of this approach, however, has been hampered by poor polymer/zeolite adhesion, inadequate particle dispersion and low membrane flux.
The developing of new nanostructured materials with specific configurations and morphology is offering powerful tools for the preparation of membranes with highly controllable selectivity and permeability for gas separation (Lai, Z. P. et al, 2003; De Vos, R. M. et al, 1998; Merekel, T. C. et al, 2002; Shiflett, M. B. et al, 1999). Up to date, nanocomposite membranes are almost keep the thickness more than hundred nanometers and with support layer, which significantly limit the membrane flux, separation efficiency and macroscale application, especially, for liquid separation system (Holt, J. K. et al, 2006; Jirage, K. B. et al, 1997). Even several ultrathin (several tens nanometers thick) free-standing films were reported (Yang, H. et al, 1996; Mamedov, A. A. et al, 2002; others), and used for sensors and actuators, but without any report about their separation performance because of the lack of the functional designation and workability, except that the first example for using ultrathin nanomembranes for size-based macromolecular separation was carried out by Striemer' and coworkers by using 15 nm thick free-standing silicon membranes prepared by using precision deposition of silicon and etching techniques and thermal annealing process at high temperature (above 700° C.) (Striemer. C. C. et al, 2007).
In our laboratory, we developed a general method to synthesize macroscale ultrathin free-standing mesoporous films with fibrous nanocomposite of negatively charged dye molecules (see non-patent ref. 1), DNA (see non-patent ref. 2), and positively charged metal hydroxide nanostrands (see non-patent ref. 3, non-patent ref. 4) by a simple filtration and peeling off techniques. Unfortunately, these fibrous nanocomposite films were fragile and easily destroyed due to the weak chemical stability of metal hydroxide nanostrands. Therefore, conjugated polymers (polyaniline, polypyrrole) was coated on nanostrands and formed mesoporous thin films for size selective separation of proteins in physiological conditions (Peng, X. S. et al, 2007). However, such film still can not be sustained in the solution with pH lower than 4.
[Non-Patent Ref. 1]
    Luo, Y.-H., Huang, J., Ichinose, I. “Bundle-like assemblies of cadmium hydroxide nanostrands and anionic dyes” J. Am. Chem. Soc. 127, 8296-8297 (2005).[Non-Patent Ref. 2]    Ichinose, I., Huang, J., Lou, Y.-H. “Electrostatic trapping of double-strand DNA by using cadmium hydroxide nanostrands” Nano Lett. 5, 97-100 (2005).[Non-Patent Ref. 3]    Ichinose, I., Kurashima, K., Kunitake. T. “Spontaneous formation of cadmium hydroxide nanostrands in water” J. Am. Chem. Soc. 126, 7162-7163 (2004).[Non-Patent Ref. 4]    Luo, Y.-H. et al. “Formation of positively charged copper hydroxide nanostrands and their structural characterization” Chem. Mater. 18, 1795-1782 (2006)